Modular mud lift pump assembly

ABSTRACT

A method and system for lifting drilling mud from subsea to a drilling vessel, which uses a pump having a body with a chamber, and a bladder in the chamber. The bladder attaches to the body and defines water and mud sides in the chamber. A mud inlet valve allows mud into the mud side of the chamber; which moves the bladder into the water side and urges water in the water side from the chamber and through a water exit valve. Pressurized water enters the chamber through a water inlet valve, which in turn pushes the bladder and mud from the chamber through a mud exit valve. The bladder separates the mud and water as it reciprocates in the chamber. The travel of the bladder in the chamber is controlled to prevent damage from contact with the chamber.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present disclosure relates in general to a modular system for pumping drilling mud from subsea to above the sea surface.

2. Description of Prior Art

Subsea drilling systems typically employ a vessel at the sea surface, a riser connecting the vessel with a wellhead housing on the seafloor, and a drill string. A drill bit is attached on a lower end of the drill string, and used for excavating a borehole through the formation below the seafloor. The drill string is suspended subsea from the vessel into the riser, and is protected from seawater while inside of the riser. Past the lower end of the riser, the drill string inserts through the wellhead housing just above where it contacts the formation. Generally, a rotary table or top drive is provided on the vessel for rotating the string and bit. Drilling mud is usually pumped under pressure into the drill string, and is discharged from nozzles in the drill bit. The drilling mud, through its density and pressure, controls pressure in the well and cools the bit. The mud also removes formation cuttings from the well as it is circulated back to the vessel. Traditionally, the mud exiting the well is routed through an annulus between the drill string and riser. However, as well control depends at least in part on the column of fluid in the riser, the effects of corrective action in response to a well kick or other anomaly can be delayed.

Fluid lift systems have been deployed subsea for pressurizing the drilling mud exiting the wellbore. Piping systems outside of the riser carry the mud pressurized by the subsea lift systems. The lift systems include pumps disposed proximate the wellhead, which reduce the time for well control actions to take effect.

SUMMARY OF THE INVENTION

Disclosed herein are examples of a system and method of lifting drilling fluid from a subsea wellbore to above the sea surface. In one example, disclosed is a system for lifting the drilling fluid from a subsea wellbore that includes a drilling riser having a return flow of the drilling fluid, a subsea module coupled with the drilling riser and having piping, and that is transportable to the drilling riser on a vessel having a capacity that is less than a capacity of a rig used in conjunction with the drilling riser. Also included is a riser module coupled with the drilling riser and having controls, and that is transportable to the drilling riser on the vessel, and a pump module coupled with the drilling riser. The pump module has a pump that is in fluid communication with the drilling fluid in the drilling riser via the piping in the subsea module and that is in communication with the controls in the riser module. The pump module is transportable to the drilling riser on the vessel. The pump module can be a first pump module, in this example the system further includes a second pump module that is symmetric and interchangeable with the first pump module. Further in this example, each pump module includes three pumps. Each pump may have a housing, a water space in the housing, a mud space in the housing that is in pressure communication with the water space, a bladder mounted in the housing having a side in contact with the water space and an opposing side in contact with the mud space, and that defines a barrier between the water and mud space. Optionally, the pump module, the subsea module, and the riser module each have a weight less than 50 metric tons. In an optional embodiment, the piping in the subsea module includes a portion for bypassing the pump module.

Also disclosed herein is an example method of lifting drilling fluid from subsea that includes providing a pump module having a series of pumps, providing a riser module having controls for the pump module, providing a subsea module having piping, forming a mud pump kit by coupling together the pump module, riser module, and subsea module, coupling the mud pump kit with a subsea riser, flowing mud from the riser to the pump module via the subsea module, and lifting the mud to above sea surface by pressurizing the mud with the pumps in the pump module. Alternatively, a second pump module can be included that is symmetric with the first pump module, so that the first and second pump modules can be interchangeable. In an example, the pump module, the subsea module, and the riser module are transported individually to a drilling riser on the sea surface with a vessel having a limited capacity. A spare pump module can be optionally provided, where the method further includes replacing the pump module with the spare pump module. The pump module can be controlled with the controls from the riser module. In one alternative, mud flow is bypassed around the pump module and through the subsea module.

Another example method of lifting drilling fluid from subsea includes providing first and second pump modules that are symmetric to one another. In this example, each of the pump modules has a series of pumps. The method further includes providing a riser module having controls for the pump modules, providing a subsea module having piping; and transporting the first pump module, the second pump module, the riser module, and subsea module on a vessel and to an offshore rig. Here, each of the pump modules, the riser module, and subsea module are individually transported to the rig on the vessel. A mud pump kit is formed by coupling together the pump modules, riser module, and subsea module on the offshore rig, and the mud pump kit is coupled with a subsea riser that is operated in conjunction with the offshore rig. Mud is flowed from the riser to the pump modules via the subsea module, and the mud is lifted to above sea surface by pressurizing the mud in the pump modules. A spare pump module can optionally be provided; where the spare pump module is used to replace one of the first or second pump modules. In one alternative, the controls in the riser module include a processor and hydraulic power units, the method of this example can further include using the processor to selectively open and close valves provided with the pump modules.

BRIEF DESCRIPTION OF DRAWINGS

Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side sectional view of an example of a subsea drilling system in accordance with the present invention.

FIGS. 2 and 3 are partial side sectional views of an example of a subsea pump for use with the drilling system of FIG. 1 in different pumping modes and in accordance with the present invention.

FIG. 4 is a side view of an embodiment of an example of a lift pump assembly in accordance with the present invention.

FIG. 5 is a side view of an alternate embodiment of the drilling system of FIG. 1 and in accordance with the present invention.

FIG. 6 is a perspective view of a portion of the drilling system of FIG. 6, and in accordance with the present invention.

While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF INVENTION

The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout.

It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation.

Shown in FIG. 1 is a side partial sectional view of an example embodiment of a drilling system 10 for forming a wellbore 12 subsea. The wellbore 12 intersects a formation 14 that lies beneath the sea floor 16. The wellbore 12 is formed by a rotating bit 18 coupled on an end of a drill string 20 shown extending subsea from a vessel 22 floating on the sea surface 24. The drill string 20 is isolated from seawater by an annular riser 26; whose upper end connects to the vessel 22 and lower end attaches onto a blowout preventer (BOP) 28. The BOP 28 mounts onto a wellhead housing 30 that is set into the sea floor 16 over the wellbore 12. A mud return line 32 is shown having an end connected to the riser 26 above BOP 28, which routes drilling mud exiting the wellbore 12 to a lift pump assembly 34 schematically illustrated subsea. Within the lift pump assembly 34, drilling mud is pressurized for delivery back to the vessel 22 via mud return line 36.

FIG. 2 includes a side sectional view of an example of a pump 38 for use with lift pump assembly 34 (FIG. 1). Pump 38 includes a generally hollow and elliptically shaped pump housing 40. Other shapes for the housing 40 include circular and rectangular, to name a few. An embodiment of a flexible bladder 42 is shown within the housing 40; which partitions the space within the housing 40 to define a mud space 44 on one side of the bladder 42, and a water space 46 on an opposing side of bladder 42. As will be described in more detail below, bladder 42 provides a sealing barrier between mud space 44 and water space 46. In the example of FIG. 2, bladder 42 has a generally elliptical shape and an upper open space 48 formed through a side wall. Upper open space 48 is shown coaxially registered with an opening 50 formed through a side wall of pump housing 40. A disk-like cap 52 bolts onto opening 50, where cap 52 has an axially downward depending lip 53 that coaxially inserts within opening 50 and upper open space 48. A portion of the bladder 42 adjacent its upper open space 48 is wedged between lip 53 and opening 50 to form a sealing surface between bladder 42 and pump housing 40.

A lower open space 54 is formed on a lower end of bladder 42 distal from upper open space 48, which in the example of FIG. 2 is coaxial with upper open space 48. An elliptical bumper 56 is shown coaxially set in the lower open space 54. The bumper 56 includes upper and lower segments 58, 60 coupled together in a clamshell like arrangement, and that respectively seal against upper and lower radial surfaces on the lower open space 54. The combination of sealing engagement of cap 52 and bumper 56 with upper and lower open spaces 42, 54 of bladder 42, effectively define a flow barrier across the opposing surfaces of bladder 42. Further shown in the example of FIG. 2 is an axial rod 62 that attaches coaxially to upper segment 56 and extends axially away from lower segment 58 and through opening 50.

Still referring to FIG. 2, a mud line 64 is shown having an inlet end connected to mud return line 32, and an exit end connected with mud return line 36. A mud inlet valve 66 in mud line 64 provides selective fluid communication from mud return line 32 to a mud lead line 68 shown branching from mud line 64. Lead line 68 attaches to an annular connector 70, which in the illustrated example is bolted onto housing 40. Connector 70 mounts coaxially over an opening 72 shown formed through a sidewall of housing 40 and allows communication between mud space 44 and mud line 64 through lead line 68. A mud exit valve 74 is shown in mud line 64 and provides selective communication between mud line 64 and mud return line 36.

Water may be selectively delivered into water space 46 via a water supply line 76 (FIG. 1) shown depending from vessel 22 and connecting to lift pump assembly 34. Referring back to FIG. 2, a water inlet lead line 78 has an end coupled with water supply line 76 and an opposing end attached with a manifold assembly 80 that mounts onto cap 52. The embodiment of the manifold assembly 80 of FIG. 2 includes a connector 82, mounted onto a free end of a tubular manifold inlet 84, an annular body 86, and a tubular manifold outlet 88, where the inlet and outlet 84, 88 mount on opposing lateral sides of the body 86 and are in fluid communication with body 86. Connector 82 provides a connection point for an end of water inlet lead line 78 to manifold inlet 84 so that lead line 78 is in communication with body 86. A lower end of manifold body 86 couples onto cap 52; the annulus of the manifold body 86 is in fluid communication with water space 46 through a hole in the cap 52 that registers with opening 50. An outlet connector 90 is provided on an end of manifold outlet 88 distal from manifold body 86, which has an end opposite its connection to manifold outlet 88 that is attached to a water outlet lead line 92. On an end opposite from connector 90, water outlet lead line 92 attaches to a water discharge line 94; that as shown in FIG. 1, may optionally provide a flow path directly subsea.

A water inlet valve 96 shown in water inlet lead line 78 provides selective water communication from vessel 22 (FIG. 1) to water space 46 via water inlet lead line 78 and manifold assembly 80. A water outlet valve 98 shown in water outlet lead line 92 selectively provides communication between water space 46 and water discharge line 94 through manifold assembly 80 and water outlet lead line 92.

In one example of operation of pump 38 of FIG. 2, mud inlet valve 66 is in an open configuration, so that mud in mud return line 32 communicates into mud line 64 and mud lead line 68 as indicated by arrow A_(Mi). Further in this example, mud exit valve 74 is in a closed position thereby diverting mud flow into connector 70, through opening 72, and into mud space 44. As illustrated by arrow A_(U), bladder 42 is urged in a direction away from opening 72 by the influx of mud, thereby imparting a force against water within water space 46. In the example, water outlet valve 98 is in an open position, so that water forced from water space 46 by bladder 42 can flow through manifold body 86 and manifold outlet 88 as illustrated by arrow A_(Wo). After exiting manifold outlet 88, water is routed through water outlet lead line 92 and into water discharge line 94.

An example of pressurizing mud within mud space 44 is illustrated in FIG. 3, wherein valves 66, 98 are in a closed position and valves 96, 74 are in an open position. In this example, pressurized water from water supply line 76 is free to enter manifold assembly 80 where as illustrated by arrow A_(Wi), the water is diverted through opening 50 and into water space 46. Introducing pressurized water into water space 46 urges bladder 42 in a direction shown by arrow A_(D). Pressurized water in the water space 46 urges bladder 42 against the mud, which pressurizes mud in mud space 44 and directs it through opening 72. After exiting opening 72, the pressurized mud flows into lead 68, where it is diverted to mud return line 36 through open mud exit valve 74 as illustrated by arrow A_(Mo). Thus, providing water at a designated pressure into water supply line 76 can sufficiently pressurize mud within mud return line 36 to force mud to flow back to vessel 22 (FIG. 1).

As illustrated in FIGS. 2 and 3, bumper 56 travels axially within housing 40, and has end strokes proximate to the inner surface of housing 40. An optional controller 100 (FIG. 1) may be provided for limiting travel of bladder 42 and bumper 56 to avoid collisions of bladder 42 or bumper 56 with the inner surface of housing 40. In an embodiment, controller 100 includes an information handling system, and receives or contains instructions to selectively operate valves 66, 74, 78, 98. Optionally, valves 66, 74, 78, 98 can include actuators (not shown) in communication with and/or controlled by controller 100, that manipulate the valves 66, 74, 78, 98 to limit travel of the bumper 56. The controller 100 can be set based upon an increase or decrease in fill volume that alters velocity of flow in one of the chambers 44, 46. User defined set points can be input to the controller 100 for establishing limits of travel of the bladder 42. This can be manifested via control of the valves 66, 74, 96, 98 so that they open and close at designated times and sequences so that travel of bladder 42 and/or bumper 56 prevents or avoids collision with housing 40. Moreover, a set bias may be included with commands in the controller so that the control system automatically adjusts the set points to a higher or lower value to bring bladder travel within a safe range and thereby avoid any damaging contact. Examples exist wherein volume in one of the chambers 44, 46 at a maximum stroke ranges from about 15 gallons to about 55 gallons. By setting the set points with an included bias, the set points are adjusted during use so that in a subsequent cycle of pumping, the extent of bladder travel is decreased to avoid any overshoot from a designated position.

FIG. 4 is a schematic illustration of an example of a lift pump assembly 34 having pumps 38A-C arranged in parallel. In this example, and similar to that of FIG. 2, mud flows to pumps 38A-C respectively from mud lines 64A-C that each have an inlet end connected to mud return line 32. Outlet ends of the mud lines 64A-C discharge into mud return line 36. Leads 68A-C respectively communicate mud flow between pumps 38A-C and lines 64A-C, where valves 66A-C, 74A-C respectively regulate flow through lines 64A-C. In similar fashion, water from water supply line 76 flows to pumps 38A-C via water inlet lead lines 78A-C and manifold assemblies 80A-C; and water from pumps 38A-C is delivered to water discharge line 94 via manifold assemblies 80A-C and water outlet lead lines 92A-C. Water to and from pumps 38A-C is controlled by valves 96A-C and 98A-C, which are shown respectively in lines 78A-C and lines 92A-C. Optionally, one or more of valves 66A-C, 74A-C, 96A-C, 98A-C, 106A-C, 108A-C may be in communication with a controller 100 for selective opening and/or closing the valves, or throttling flow through the valves.

The lift pump assembly 34 of FIG. 4 is equipped with a pressure balance circuit 102 for minimizing a pressure differential across valves 96A-C. In the example of FIG. 4, pressure balance circuit 102 includes pressurization tubing 104A-C, each having inlets respectively connected to water inlet lead lines 78A-C. Optionally, pressurization tubing 104A-C can connect directly to water supply line 76. Pressurization valves 106A-C are provided within each run of pressurization tubing 104A-C. Each run of tubing 104A-C includes depressurization valves 108A-C downstream of pressurization valves 106A-C. Tubing leads 110A-C branch respectively from pressurization tubing 104A-C in the portions between pressurization valves 106A-C and depressurization valves 108A-C. The ends of tubing 110A-C distal from pressurization tubing 104A-C connect to water inlet lead lines 78A-C downstream of inlet valves 96A-C. In an example of operation, when water is being discharged from pumps 38A-C, outlet valves 98A-C are in the open position, and inlet valves 96A-C are in the closed position, a pressure differential can exist across inlet valves 96A-C that can approach pressure in water supply line 76. Further in this example, opening valves 106A-C, while valves 96A-C and 108A-C are in a closed position, communicates pressure from line 76 through pressurization tubing 104A-C, tubing leads 110 A-C, and into inlet lead lines 78A-C downstream of valves 96A-C. In this example embodiment, fluid in lines 78A-C upstream and downstream of valves 96A-C is in pressure communication with line 76, thereby minimizing pressure differential across valves 96A-C.

Downstream of valves 108A-C, pressurization tubing 104A-C connects to a tubing header 112, through which water in the pressure balance circuit 102 can be discharged to ambient. In the example of FIG. 4, pumps 38A-C and the associated piping disclosed herein are referred to as a pump module 114A. Example embodiments exist wherein the lift pump assembly 34 includes two or more modules. As such, a water discharge line 116 from another module 114B, that is substantially similar to module 114A. Block valves 118, 120 are respectively provided in discharge lines 94, 116 for isolating water flow from modules 114A, 114B. Also in line 94 is an optional block valve 122 downstream of the intersection of line 116 with line 94; and a control valve 124 and flow meter 126 downstream of block valve 122. An optional bypass line 128 connects tubing header 112 to water discharge line 94 between control valve 124 and flow meter 126. A block valve 130 is shown in tubing header 112 downstream of bypass line 128, and a block valve 132 is provided in bypass line 128. In an alternative embodiment, block valves 130, 132 are in communication with controller 100.

Still referring to the example of FIG. 4, line 94 discharges to ambient downstream of control valve 124, thus depending on the flow rate of fluid in line 94, pressure in line 94 downstream of control valve 124 is substantially equal to ambient pressure. In the illustrated embodiment, control valve 124 and flow meter 126 are shown in communication with one another, so that a flow area through control valve 124 automatically adjusts in response to a flow rate detected by flow meter 126 to “throttle” flow across control valve 124. Optionally as shown, control valve 124 is in communication with controller 100, so that the amount of throttling can vary based on operating conditions of the lift pump assembly 34. As such, a pressure differential can be generated across control valve 124 so that pressure in line 94 upstream of control valve 124 is greater than pressure at ambient and introduces a backpressure in line 94. Where the backpressure in line 94 suppresses flow rate spikes in lines 92A-C, which in turn reduces cycling forces on components of pumps 38A-C during pumping operations.

In some examples of use, pumps 38A-C operate under “managed pressure drilling operations” where mud flow rates are reduced, but pressure of the mud to the pumps 38A-C is increased. During these conditions, the flow path to ambient through the pressure balance circuit 102 and from lines 78A-C can allow pressure in pumps 38A-C to drop below a threshold value so that pumps 38A-C will uncontrollably fill with mud during a subsequent pumping cycle. One example of operation to address the unacceptable pressure drop includes diverting flow in tubing header 112 that is being discharged from pressure balance circuit 102 through bypass line 128. In this example, block valve 130 is set into a closed position and block valve 132 is open. In an optional example, controller 100 delivers instructions for opening/closing of the block valves 130, 132. As indicated above, bypass line 128 terminates into water discharge line 94 upstream of control valve 124, which is maintained at a pressure sufficiently above ambient so that a backpressure can be exerted onto pressure balance circuit 102. In the example of FIG. 4, the backpressure on the pressure balance circuit 102 communicates to the water side 46 (FIG. 2) of each pump 38A-C; which maintains a minimum pressure in the water side 46 of each of the pumps 38A-C to avoid an uncontrolled influx of mud flow into the pumps 38A-C.

Referring now to FIG. 5, an alternate embodiment of drilling system 10A is shown in side partial sectional view and wherein lift pump assembly 34A includes a mud pump kit 134 mounted integral onto riser 26A. In this example, mud pump kit 134 includes a subsea module 136 shown circumscribing riser 26A and that includes mud distribution manifold (not shown) and other flow control devices for selectively diverting flow to desired destinations. A riser module 137 is illustrated mounted on an upper surface of subsea module 136, which also circumscribes riser 26A. Riser module 137 of FIG. 4 includes controls for operation of the pump kit 134, such as a processor 138 having hardware and software for controlling operation of components of pump kit 134. Also included in riser module 137 are hydraulic power units 139 for providing pressurized hydraulic fluid, which in an example is used for actuating devices subsea, such as valves in pump kit 134. Riser module 137 also includes hydraulic control systems connection hardware for mounting mud pump kit 134 to riser 26A. Pumps 38 (FIG. 2) are housed in pump modules 140, 142 shown set on riser module 137. In an embodiment, pump modules 140, 142 each include three pumps 38. In an example, included in the subsea module 136 is piping 143 that provides connectivity, and communication of mud flow, between the riser 26A and pump modules 140, 142. Piping 143 further alternatively includes a circuit that connects to riser 26A, but bypasses pump modules 140, 142. Examples of operation where pump modules 140, 142 are bypassed include situations where pressure in the mud flow is sufficient for flowing to surface, or where pump module(s) 140, 142 are not in service. In an example, valves 144 (that can be part of a valve kit) in the pump modules 140, 142 are actuated and/or controlled by processor 138, and may optionally be powered by hydraulic fluid supplied from hydraulic power unit 139.

A solids recovery unit (SRU) 145 is shown above the pump modules 140, 142, and a subsea rotating device (SRD) 146 attaches to an upper end of SRU 145. An upper end of SRD 146 flangedly attaches to a riser joint 148; where in one example a substantial portion of the riser 26A between SRD 146 and vessel 22 (FIG. 1) is made up of stacked riser joints 148.

In the example of FIG. 4, mud exiting drill string 20 flows upward in an annulus 150 defined between drill string 20 and wellbore 12, and which extends further upward between drill string 20 and riser 26A. The mud flows past mud pump kit 134 and SRU 145 within annulus 118 and into SRU 146 where a packer (not shown) blocks the mud. In an embodiment, the annulus 118 above packer is filled with sea water or other fluid. Mud within annulus 118 below packer is diverted to SRU 145 where cuttings or other solids are removed or particulated. After being processed in the SRU 145, the mud is directed to the pump modules 140, 142 where it is pressurized so it can flow back to vessel 22. Processing the mud in the SRU 145 can prevent damage to the pumps 38 (FIG. 2) in the modules 140, 142.

In an example, modules 136, 137, 140, 142 are modular elements that can be transported separately to the vessel 22 (FIG. 1) on site, where the pump kit 134 is assembled. Still referring to FIG. 1, a vessel 152, which in an example is smaller than vessel 22, is shown transporting a module 136 to vessel 22. Optionally, embodiments exist where none of the modules 136, 137, 140, 142 weigh in excess of 50 metric tons. A maximum weight of 50 metric tons is advantageous as this is the upper weight capacity of most barges. In alternatives where the weight of each of modules 136, 137, 140, 142 does not exceed 50 metric tons, any one of modules 136, 137, 140, 142 can be individually transported to vessel 22 with vessel 152. A significant time savings is one advantage of the modularity of modules 136, 137, 140, 142. Due to the weight of the pump kit 134, and that the pump kit 134 asymmetrically loads an offshore rig or vessel 22, which requires anchoring and stabilization, loading a fully assembled pump kit 134 onto a vessel 22 is impractical. Whereas vessel 152 can transport the modules 136, 137, 140, 142 individually, and vessel 22 can accommodate individual modules 136, 137, 140, 142 on site and without becoming unstable.

In an optional embodiment, pump modules 140, 142 are individually detachable from the pump kit 134, and thus further enhancing modularity of the pumping system. Dedicated piping (not shown) may be routed from SRU 145 and separately to each module 140, 142 so that one of the modules 140, 142 can remain operational while the other is removed or otherwise out of service. Further, spare modules can be kept on site for one or both modules 140, 142, and can installed in place of a one of the modules 140, 142 with little or no stoppage of operation of pumping mud to the vessel 22.

Yet further optionally, BOP 28A is a BOP stack, whose upper portion includes an annular blowout preventer and is part of a lower marine riser package (LMRP). Additionally, LMRP can include controls, a multiplexer unit, and pods. In an embodiment, modules 136, 137, 140, 142, SRU 145, SRU 146, BOP 28A, and riser joints 116 are delivered to the vessel 22 (FIG. 1) while on site and disposed above wellbore 12. While on the vessel 22, modules 136, 137, 140, 142 are attached together to form mud pump kit 134 which is coupled with BOP 28A. SRU 145 and SRU 146 are attached onto mud pump kit 134; while suspended from riser joints 116 the assembled unit is lowered subsea onto wellhead housing 30.

Referring now to FIG. 6, shown in a perspective view is a schematic example of a mud pump kit 134 mounted onto a riser 26A. As shown, pump modules 140, 142 are stacked respectively on starboard and port sides of riser 26A and on top of riser module 137; where riser module 137 stacks on top of subsea module 136. As illustrated in FIG. 6, mud return line 36 shown including a fitting 156 between where mud return line 36 couples with riser 26A and the pumps 38A-C. Fitting 156 can be a flanged surface, a valve, or any other device for fluidically coupling sections of a line. In this example pump modules 140, 142 are symmetric to one another so that pump module 140 can be switched out for pump module 142 (and vice versa). Thus the corresponding mud return line (not shown), provided with pump module 142 and for pumps 38D-F, is oriented to mate with fitting 156. The symmetric/mirror image configuration of pump modules 140, 142 allows one of the pump modules 140, 142 to be switched out for the other without rearranging any piping. An advantage of this design is that only a single spare pump module 160 need be stored onsite, which can be used to replace either of pump module 140 or pump module 142.

The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims. 

What is claimed is:
 1. A system for lifting drilling fluid from a subsea wellbore comprising: a drilling riser having a return flow of the drilling fluid; a subsea module coupled with the drilling riser and having piping, and that is transportable to the drilling riser on a vessel having a capacity that is less than a capacity of a rig used in conjunction with the drilling riser; a riser module coupled with the drilling riser and having controls, and that is transportable to the drilling riser on the vessel; and a pump module coupled with the drilling riser, and having a pump that is in fluid communication with the drilling fluid in the drilling riser via the piping in the subsea module and that is in communication with the controls in the riser module, and that is transportable to the drilling riser on the vessel.
 2. The system of claim 1, wherein the pump module comprises a first pump module, the system further comprising a second pump module that is symmetric and interchangeable with the first pump module.
 3. The system of claim 2, wherein each pump module comprises three pumps.
 4. The system of claim 3, wherein each pump comprises a housing, a water space in the housing, a mud space in the housing that is in pressure communication with the water space, a bladder mounted in the housing having a side in contact with the water space and an opposing side in contact with the mud space, and that defines a barrier between the water and mud space.
 5. The system of claim 1, wherein the pump module, the subsea module, and the riser module each have a weight less than 50 metric tons.
 6. The system of claim 1, wherein the piping in the subsea module includes a portion for bypassing the pump module.
 7. A method of lifting drilling fluid from subsea comprising: providing a pump module having a series of pumps; providing a riser module having controls for the pump module; providing a subsea module having piping; forming a mud pump kit by coupling together the pump module, riser module, and subsea module; coupling the mud pump kit with a subsea riser; flowing mud from the riser to the pump module via the subsea module; and lifting the mud to above sea surface by pressurizing the mud with the pumps in the pump module.
 8. The method of claim 7, wherein the pump module comprises a first pump module, the method further comprising providing a second pump module that is symmetric with the first pump module, so that the first and second pump modules are interchangeable.
 9. The method of claim 7, wherein the pump module, the subsea module, and the riser module are transported individually to a drilling riser on the sea surface with a vessel having a limited capacity.
 10. The method of claim 7, further comprising providing a spare pump module, and replacing the pump module with the spare pump module.
 11. The method of claim 7, wherein the pump module is controlled with the controls from the riser module.
 12. The method of claim 7, further comprising bypassing mud flow around the pump module and through the subsea module.
 13. A method of lifting drilling fluid from subsea comprising: providing first and second pump modules that are symmetric to one another, each of the pump modules having a series of pumps; providing a riser module having controls for the pump modules; providing a subsea module having piping; individually transporting the first pump module, the second pump module, the riser module, and subsea module on a vessel and to an offshore rig; forming a mud pump kit by coupling together the pump modules, riser module, and subsea module on the offshore rig; coupling the mud pump kit with a subsea riser that is operated in conjunction with the offshore rig; flowing mud from the riser to the pump modules via the subsea module; and lifting the mud to above sea surface by pressurizing the mud in the pump modules.
 14. The method of claim 13, further comprising providing a spare pump module, and replacing one of the first or second pump modules with the spare pump module.
 15. The method of claim 13, wherein the controls in the riser module include a processor and hydraulic power units, the method further comprising using the processor to selectively open and close valves provided with the pump modules. 